When fileting fish, it is fundamentally important to maximize the yield of valuable filet meat and simultaneously to prevent undesired skeletal parts from remaining in the filet. If skeletal parts remain in the filet, expensive trimming work following the filet production is necessary to remove such parts from the filet meat. This trimming work is expensive particularly because only qualified personnel is eligible, which performs this processing manually.
A prerequisite for a filet production optimized in both senses is, among others, that the fish are aligned precisely in the position of their line of symmetry with respect to the fileting tools, so that cutting away the dorsal and ventral bones with narrowly spaced tools can take place without danger of skeletal parts being cut into.
In a known process, this problem is confronted in that the unpaired fins and fin-holders are first removed and then the fileting cuts for cutting away the dorsal and ventral bones are made. Such a process is known, for instance, from GB 466 674, in which, for fish conveyed in their longitudinal direction, incisions are made from both sides of the dorsal fins by cutters positioned at an angle, so that a strip of flesh comprising the fins and the fin-holder is cut out of the fish carcass.
For fish with an ordinary arrangement and formation of fins, this process leads to satisfactory fileting results, but cannot be applied to fish of the Siluridae species. The cause for this is the presence of a stable dorsal spike in front of the first dorsal fin and anchored by an internal column of bone to the front end of the spinal column and supported by a bone plate situated directly under the skin that holds the dorsal spike in place on the cranial roof.
In an ordinary back fileting cutting tool consisting of a pair of knives whose spacing is adapted to the width of the back ribs, the bone plate on either side of the dorsal spike is cut through, so that the outer parts of the bone plate remain in the filet meat and thus the aforementioned trimming work is necessary.